<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        #triangle-up {
            width: 0;
            height: 0;
            border-left: 50px solid transparent;
            border-right: 50px solid transparent;
            border-bottom: 100px solid red;
        }
    </style>
</head>
<body>
<div id="triangle-up"></div>
</body>
<script>

</script>
</html>